How long does it generally take Apple to propagate subject category changes in my submitted eBooks

I have several books on sale in the ibooks store. A couple of weeks ago I did a minor change to the subject category of all my books. About a week after that I started to notice a drop on sales. I am trying to figure out whether this drop might be due to normal seasonal cycles or it might be that my change got propagated and classified the books in the wrong pile.